Need help in understand C code

Question

Hi I need some help in understand some C code:

#if 0
   some C code 1
#elif 0
   static int8 arry[10];
#pragma Align_to(32, arry)
   ASSERT(((int8ptr_t)arry) & 15) == 0)
#else
   ASSERT(((int8ptr_t)arry) & 15) == 0)
#endif

My questions:

Is only the #else part compiled?
What is the meaning of #pragma Align_to(32, arry) in the #elif 0 case?

I believe it's #pragma. gcc.gnu.org/onlinedocs/cpp/Pragmas.html — Evan Mulawski
– Evan Mulawski, Commented Jun 4, 2013 at 13:15
Whether it's pragma or pramga isn't relevant... the #elif 0 means it is ignored. Only the #else is compiled as the OP surmises. — K Scott Piel
– K Scott Piel, Commented Jun 4, 2013 at 13:18
@KScottPiel Is that required? I've known compilers that process all #pragmas irrespective of whether or not they're in a #if — Tom Tanner
– Tom Tanner, Commented Jun 4, 2013 at 13:44

Dewfy · Accepted Answer · 2013-06-04 13:18:33Z

2

Actually better way to answer is ask compiler - use g++ -E or MSVC: cl /EP to print what is really compiled

answered Jun 4, 2013 at 13:18

Dewfy

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3 Comments

Jens Over a year ago

Programming by experimentation? I'm not convinced that asking a compiler will give any definitive answer--unlike reading the C language spec/Standard will.

Dewfy Over a year ago

@Jens my answer related to first part of question - instead of solving puzzle "what part of preprocessor monster is compiled" I'd prefer to generate expanded file

Jens Over a year ago

+1 Fair enough, but we need to be aware of the experimentation pitfalls in general.

chux · Accepted Answer · 2013-06-04 15:35:04Z

0

Yes, the #else part is what is compiled.

The #pragma directive is a compiler specific directive. As you compiler was not specified, it could mean anything.

In your case #pragma Align_to(32, arry), likely tells the compiler to insure variable 'arry' is place in memory on a 32 byte boundary. This is typically for performance reasons or compatibility concerns. You may also want to look into the keyword __attribute__ use to control similar variable attributes.

answered Jun 4, 2013 at 15:35

chux

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Comments

Jens · Accepted Answer · 2013-06-04 13:42:48Z

0

Answer to 1: Yes, but note that even the parts within #if 0 etc. must consist of valid preprocessing tokens. This means that this will fail with a diagnostic:

#if 0
That's what C is all about
#endif

because there's an unterminated character constant introduced by the lone '. Same goes for unterminated string literals.

Answer to 2: The pragma is a hint to the compiler that the address of arry shall be aligned on a multiple of 32.

edited Jun 4, 2013 at 13:42

answered Jun 4, 2013 at 13:22

Jens

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